1. Field of the Invention
The present invention relates generally to radio geolocation and particularly to cooperative receivers for position location using periodic codes in such broadcast digital transmissions as broadcast digital television (DTV) and wireless local area network (WLAN) signals.
2. Description of the Prior Art
Among different radio geolocation and navigation systems, there are two important systems in wide use today. One is the 100 kHz Long Range Navigation-C (LORAN-C) system which evolved to the present form in the mid-1950s. It uses terrestrial radio transmitters to provide navigation, location, and timing services for suitably equipped air, land and marine users, civil and military alike. A LORAN-C receiver measures the difference in times of arrival of pulses transmitted by a chain of three to six synchronized transmitter stations separated by hundreds of kilometers. There are many LORAN chains of stations around the globe. Modernization effort is underway to enhance the accuracy, integrity, availability, and continuity of the LORAN system, known as Enhanced LORAN or eLORAN for short.
The other is the increasingly popular satellite-based Global Positioning System (GPS). Fully operational since 1994, the GPS relies upon a nominal constellation of twenty-four satellites in six different orbit plans around the Earth for position location, navigation, survey, and time transfer. Each satellite carries a set of ultra precise atomic clocks and transmits pseudo-noise (PN) code modulated signals at several frequencies. By tracking four or more satellites, a user can solve for the variables of longitude, latitude, altitude and time to precisely determine the user's location and calibrate its clock. More details are provided in the books entitled, Global Positioning System: Theory and Applications (Vols. I and II), edited by B. W. Parkinson and J. J. Spilker Jr., AIAA, 1996; Understanding GPS: Principles and Applications, edited by E. D. Kaplan, Artech House Publishers, 1996; Fundamentals of Global Positioning System Receivers—A Software Approach, by J. B. Y. Tsui, John Wiley & Sons, Inc., 2000; and Global Positioning System, Signals, Measurements, and Performance, by P. Misra and P. Enge, Ganga-Jamuna Press, 2001.
Despite of its increased popularity, GPS cannot function well when the line-of-sight (LOS) view between a receiver and a GPS satellite is obstructed due to foliage, mountains, buildings, or other structures. To satisfy the requirements of location-based mobile e-commerce and emergency call location (E911), there have been ongoing efforts so as to improve GPS receiver sensitivity to operate on GPS signals of very low power level. One example is the assisted GPS (AGPS). The AGPS approach relies upon a wireless data link to distribute, in real time, such information as time, frequency, navigation data bits, satellite ephemeredes, and approximate position as well as differential corrections to special GPS receivers equipped with a network modem so as to reduce the uncertainty search space, to help lock onto signals, and to assist navigation solution. This approach, however, comes with a price associated with installing and maintaining the wireless aiding infrastructure and services.
GPS cannot function well either when GPS signal is heavily jammed or overwhelmed by unintentional interference. GPS signals may be turned off altogether from newer GPS satellites with flexible power and flexible signal capabilities when it orbits over certain region. In such circumstances, no GPS solution is available.
Amid the process of replacement of National Television System Committee (NTSC) analog television signals by an Advanced Television Systems Committee (ATSC) digital television (DTV) signal, there has been a considerable amount of efforts devoted to the use of DTV signals for position location, thus serving as a complement to and/or a substitute for GPS. This is exemplified by the U.S. Pat. No. 6,861,984, entitled, Position Location Using Broadcast Digital Television Signals, by M. Rabinowitz and J. J. Spilker Jr., issued Mar. 1, 2005.
Designed primarily for indoor reception, DTV signals exhibit several advantages. It is much higher in power (40 dB over GPS) and at lower and more diverse frequencies (nearly half of the spectrum between 30 MHz and 1 GHz). The geometry offered by a network of terrestrial DTV transmitters is superior to what a satellite constellation can provide. As such, it has better propagation characteristics with greater diffraction, larger horizon, and stronger penetration through buildings and automobiles. DTV signals have a bandwidth of 6 to 8 MHz, which is much wider than the primary lobe of GPS C/A-code (2 MHz), thereby minimizing the effects of multipath and permitting higher accuracy tracking.
With DTV transmitters fixed on the ground, their lines of sight to a user changes very slowly, only adding a small amount of Doppler shift to a DTV signal frequency. This allows the signal to be integrated over a long period of time, thus easing the task of acquisition and tracking of a weak signal considerably. As a further benefit, the component of a DTV signal that can be used for timing is of low duty factor (e.g., 1 of 313) in contract to GPS wherein the ranging code is repeatedly transmitted and has to be tracked continuously.
However, one inherent technical difficulty faced by position location using broadcast digital transmissions (BDT) such as DTV signals is the clock bias and drift of the signal timing source at a transmitter, which are unknown to a user. Although it may be possible to have all DTV stations use ultra-precise atomic clocks or GPS-disciplined clocks, the synchronization of all signal transmissions across a large region is a daily operational challenge. It may also be possible to time-tag all transmissions and embed the clock offset information in the broadcast signals for all stations in a given region. However, these approaches require coordinated involvement of local DTV operators who are in broadcasting and not time transfer business.
Many inventions exemplified by the U.S. Pat. No. 6,861,984 by M. Rabinowitz and J. J. Spilker Jr. mentioned earlier make use of base stations, location servers, and monitor units to calibrate the DTV transmitter timing biases and to provide the calibration data to mobile users via dedicated data links. The position location mechanism in such inventions is referred to as “reference-aiding,” wherein the signal source timing errors are estimated explicitly at the reference station and sent to users (a parametric approach) or the measurement difference is employed to remove the timing errors common to the reference station and users (a non-parametric approach). There is a significant cost associated with installing and maintaining the infrastructure of base stations, location servers, and monitor units on a large scale. A user has to subscribe to a service coverage in addition to special equipment for the service signals.
Clearly, a user is subject to the potential risk of service discontinuity when moving from one region (or a country for the matter) to another without a global service network in place or a valid global subscription. These prior-art approaches further prevent broadcast digital transmission (BDT) signals from being used for military applications as signals of opportunity (SOOP) because of lack of pre-surveyed reference/monitor units. In the U.S. Pat. No. 7,388,541, entitled “Self-Calibrating Position Location Using Periodic Codes in Broadcast Digital Transmissions,” issued Jun. 17, 2008 to the present inventor, two position location mechanisms, referred to as “self-referencing” and “self-calibrating,” respectively, are disclosed by which position location systems can make use of broadcast digital transmissions such as DTV and WLAN signals without requiring the service from external base stations, location servers, and monitor units. However, these self-aiding methods, in contrast to the above-mentioned reference-aiding methods, are useful for one user at a time and may require long time to complete the self-referencing and/or self-calibrating process.
Applications arise wherein a team of cooperative mobile users need to know not only their own location but also those of their teammates without relying upon GPS signals. By cooperative, we mean the teammates have a means to communicate to one another via a wireless data link to coordinate their activities, exchange data, and perform mutual aiding in the form of cooperative referencing and calibration. This need is met by the present invention as described and claimed below.